Matthias Wilke wrote:
> I believe, that the lens is this good.
I didn´t wanted to question your description, some friends of me had an
1000mm/10 lenses of the same manufactor and were disapointed about the
missing contrast and sharpness.
> But of course, pictures on Sensia 100 or
> Elite 100 are less contrasty.
Your sample picture on Elite looks contrasty (and more natural too IMHO)
I found several sources in the web regarding amateur astronmy which
agreed, that mirror lenses are less contrasty than refractors. Reason
should be the middle mirror and the limited reflectibility of mirrors
(90%max). They mentioned also the 1000mm/10 "Russentonnen" (Russian
-drums,-barrels), the problem of this lenses should be to tight fixing
of the mirrors and lenses resulting in missadjusted optical systems.
> My longest Zuiko is 200mm 1:4 (excellent lens),
Can only agree, it´s still good, if used with the 2x converter.
> I compared the resulting exposure times of the Rubinar and the Tokina AT-X
> 150-500mm at 1:5,6 and 500mm. The time was circa 1/10 times longer with the
> Rubinar, which may be a result of the lower light transmission of mirror
> lenses.
> So the effective aperture is maybe (say) 1:5,7 - 1:5,8.
>John P wrote:
> >It took me a while to find it - the topic of T-stops was a recent discussion
> >among the astrophotographers. Without going into the arithmetic, the T-stop
> >for the Rubinar is f/6.68. This assumes 100% reflectivity and 100%
> >transmission by the mirrors and lenses. FWIW, I get a 31percent central
> >obstruction for a primary diameter of 90mm and a central obstruction of
> >50mm.
If the mirrors reflect only 90 0ght than the light will be reduced
further from 100 0ght only 810ercent of the light will pass the
mirrors.
It´s remarkable that the refractive lens was just 1/10 stop better, so
a lot of the light is absorbed in this zooms (or the apertures are not correct).
Regards
Richard
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